Human Epidermal Growth Factor Receptor 2 (HER2), encoded by the ErbB2 gene, is a member of the epidermal growth factor receptor (EGFR/ErbB) family (Herbst, Int. J. Radiat. Oncol. Biol. Phys. 59:1-6 (2004)). HER2 is structurally similar to other EGFR family members, including HER1 (EGFR, ErbB1), HER3 (ErbB3), and HER4 (ErbB4), and also acts as a receptor tyrosine kinase. Homodimerization of HER1 and HER4 upon ligand binding activates intrinsic, intracellular protein-tyrosine kinase activity, resulting in receptor autophosphorylation and downstream signaling. These include signaling pathways such as the phosphatidylinositol 3-kinase (PI3K), the c-Jun NH(2)-terminal kinase (JNK), and the mitogen-activated protein kinase (MAPK), which promote DNA synthesis, cell proliferation, and inhibition of cell apoptosis. HER3 does not have a tyrosine kinase domain, so it transfers signals upon ligand binding through heterodimerization with other EGFR family members that have kinase activity.
Unlike HER1, HER3, and HER4, HER2 is unable to bind ligands and form homodimers. However, HER2 possesses tyrosine kinase activity, and appears to be the major signaling partner for EGFR family members through the formation of heteromeric complexes (Olayioye, Breast Cancer Res. 3:385-9 (2001)). Heterodimerization between two EGFR family members requires ligand binding (Spivak-Kroizman et al., J. Biol. Chem. 267:8056-63 (1992); Ferguson et al., Mol. Cell. Biol. 11:507-17 (2003)), but the crystal structure of a truncated HER2 ectodomain suggests that HER2 is constitutively in the activated conformation and readily interacting mostly with HER3 and other EGFR family members (Garrett et al., Mol. Cell. 11:495-505 (2003)). Overexpression of HER2 promotes ligand-independent formation of a HER2/HER3 receptor complex, a major oncogenic driver in HER2-overexpressing breast tumor cells (Stern, Sci. Transl. ed. 4:127rv2 (2012)). Cleavage of HER2 by the extracellular protease, ADAM10, produces the HER2 ectodomain and a truncated, constitutively active HER2 receptor (p95HER2) shown to drive carcinogenesis (Liu et al., Cancer Biol. Ther. 5:648-56 (2006)). HER2 overexpression is associated with strong activation of the PI3K pathway, which stimulates cell proliferation by activating the protein kinase Akt and down-regulating the cyclin-dependent kinase (CDK) inhibitor, p27 (Lane et al., Mol. Cell. Biol. 20:3210-23 (2000)). HER2 can also activate the MAPK pathway via interaction with SHC and GRB2 adaptor proteins (Dankort et al., J. Biol. Chem. 276:38921-8 (2001)). Overexpression of HER2 was found in breast and ovarian cancers, and associated with cancer metastasis (Abu Hejleh et al., World J. Gastrointest. Oncol. 4:103-8 (2012); Geng et al., Biomed. Pharmacother. 66:419-24 (2012); Nanni et al., PLoS One 7:e39626 (2012)), poor clinical outcome, and decreased survival rate (Ravdin and Chamness, Gene 159:19-27 (1995); Slamon et al., Science. 235:177-82 (1987); Slamon et al., Science 244:707-12 (1989)).
Insulin-like Growth Factor Receptor type I (IGF-IR) is a tyrosine kinase receptor composed of two α subunits and two β subunits. Upon binding to either of the two ligands, Insulin-like growth factor I (IGF-I) or IGF-II, the extracellular domain of the α chains induces tyrosine autophosphorylation of the β chains in the cytoplasm. This activates the kinase activity of IGF-IR, and triggers downstream signaling via the PI3K/Akt and Ras/MAPK pathways, resulting in increased cell survival and cell proliferation (Jones and Clemmons, Endocrine reviews 16:3-34 (1995); LeRoith et al., Endocrine reviews 16:143-63 (1995)). Elevated IGF-IR is found in many tumor malignancies, including breast, prostate, and lung cancers (Warshamana-Greene et al., Clin. Cancer Res. 11:1563-71 (2005); Jones et al., Endocr. Relat. Cancer 11:793-814 (2004)). Additionally, overexpression of IGF-IR has been associated with disease progression and cancer metastasis (Krueckl et al., Cancer Res. 64:8620-9 (2004); Yao et al., N. Engl. J. Med. 357:39-51 (2007)).
HER2 is a widely used diagnostic marker and validated target for therapy. The humanized anti-HER2 mAb Herceptin (trastuzumab) has been effective in treating HER2-overexpressing breast cancers (Hudis, N. Engl. J. Med. 357:39-51 (2007); Tokunaga et al., Int. J. Clin. Oncol. 11:199-208 (2006)). Binding of Herceptin to HER2 causes internalization and degradation of the receptor in SKBR3 and MDA453 cells (Cuello et al., Cancer Res. 61:4892-900 (2001)). Herceptin binds to domain IV of the extracellular segment of HER2, leading to disruption of HER2/HER3 dimerization and ablation of downstream PI3K/Akt signaling (Stern, Sci. Transl. ed. 4:127rv2 (2012); Kute et al., Cytometry A. 57:86-93 (2004)). Herceptin can also inhibit cleavage of HER2 ectodomain in breast cancer cells, thus blocking the generation of a constitutively active truncated receptor (p95HER2) (Liu et al., Cancer Biol. Ther. 5:648-56 (2006); Albanell et al., Adv Exp Med. Biol. 532:253-68 (2003); Molina et al., Cancer Res. 61:4744-9 (2001)). In addition, Fc-mediated antibody-dependent cellular cytotoxicity (ADCC) may partially contribute to the anti-cancer activity of Herceptin in vivo (Clynes et al., Nat. Med. 6:443-6 (2000)).
Only 25-30% of breast cancer patients overexpress HER2, and patients treated with Herceptin can develop resistance as the disease progresses. Various mechanisms may account for this resistance, which likely involves the PI3K/Akt pathway, including elevated HER2-associated receptors and other receptors (Curr. Pharmacogenomics Person. Med. 7:263-74 (2009); Nahta et al., Nat. Clin. Pract. Oncol. 3:269-80 (2006)), cross activation between HER2 and other receptors (Yarden and Sliwkowski, Nat. Rev. Mol. Cell. Biol. 2:127-37 (2001); Huang et al., Cancer Res. 70:1204-14 (2010); Nahta et al., Cancer Res. 65:11118-28 (2005)), blockage of Herceptin by membrane-associated glycoproteins such as mucin-4, removal of the Herceptin epitope by cleavage, loss of HER2 expression, or increased HER2 expression. Accumulating evidence shows that crosstalk between HER2 and IGF-IR, including receptor heterodimerization and transactivation, and elevated IGF-IR signaling are associated with Herceptin resistance (Huang et al., Cancer Res. 70:1204-14 (2010); Jin and Esteva, J. Mammary Gland. Biol. Neoplasia 13:485-98 (2008); Bender and Nahta, Front. Biosci. 13:3906-12 (2008); Casa et al., Front. Biosci. 13:3273-87 (2008)).
Overexpression of IGF-IR in HER2-overexpressing breast cancer cell lines results in Herceptin resistance in vitro (Lu et al., J. Natl. Cancer Inst. 93:1852-7 (2001)). Inhibition of IGF-IR activity enhances the response to Herceptin in HER-2-positive breast cancer cells (Browne et al., Ann. Oncol. 22:68-73 (2011)). A phase II clinical trial of HER2-positive breast cancer patients revealed that overexpression of IGF-IR in the primary tumor was associated with resistance to Herceptin (Harris et al., Clin. Cancer Res. 13:1198-207 (2007)). We previously described a human/mouse chimeric mAb m590 that specifically bound with high affinity to IGF-IR and blocked the binding of IGF-I and IGF-II. This inhibited ligand-induced phosphorylation of IGF-IR in breast cancer MCF-7 cells (Zhang et al., MAbs. 1:475-80 (2009)).